1. Field of the Invention
The invention concerns an illuminating device for the variable uniform illumination of an image field using a bent optical, homogeneous conductor and a source of light which is reproduced by means of an optical device on the inlet surface of the optical conductor.
2. Description of the Prior Art
Devices to uniformly illuminate an image field are known in themselves. Thus, in U.S. Pat. No. 2,604,005 there is described an installation for producing a uniform light flux from an arc lamp using a molded body provided with an inwardly acting reflecting mantle surface. The inlet area for the body which modifies the light flux of the arc lamp is larger than its outlet area. By means of reflection within the body, a certain equalization of brightness over the cross section of the flux of light is obtained.
U.S. Pat. No. 2,942,099 describes an installation whereby the light of an elongated lamp is concentrated on a small round spot and made uniformly bright over the entire area of the spot. Here again, a molded body with an inwardly acting reflecting mantle surface is utilized. The molded body has the shape of two truncated cones having their bases set up against each other. The cross section of the inlet area for the flux of light here is again larger than the outlet cross section. The intended uniformity of the exiting light flux is obtained by means of multiple reflections of the incoming light by the reflecting mantle surface.
Another conductor of light with a parabolically curved reflecting mantle surface is known from DE-PS No. 42 818. The source of light is arranged without imaging on the tip of the descending branch of the parabola. The beam of light is uniformly distributed on the outlet surface of the light flux.
The disadvantage of these known illuminating devices consists of the fact that specially shaped molded bodies are used which are therefore expensive to produce and require a relatively large space. They are therefore unsuited for devices wherein space must be conserved. Adjustments of the intensity of the illumination are possible only by means of the variation of the brightness of the lamp.
U.S. Pat. No. 3,926,501 describes an illuminating device wherein a bundle of optical conductors is provided, and supported in one focus of a rotationally symmetrical ellipsoid mirror. The lamp producing the light is located in the other focus of said ellipsoid. This arrangement also provides a certain degree of light uniformity over the cross section of the bundle of light beams leaving the bundle of optical conductors. Additionally, however, the brightness of the exiting light beam may be varied by rotating the ellipsoidal mirror relative to the inlet area of the optical conductor bundle so that the connecting line between the lamp and the inlet surface area encloses different angles with the longitudinal axis of the optical conductor bundle.
This known device thus again uses a special structural part expensive to produce, in the form of the ellipsoid mirror, which additionally requires considerable space. Because the bundle of optical conductors consist of numerous single fibers, it is necessary with respect to the control of brightness and the reproduction of the source of light to illuminate all of the individual fibers uniformly on the side of the light entrance, since otherwise the uniformity of the illumination on the outlet side would be adversely affected.
A prior art optical conductor is described in DE-OS No. 2 012 100. The optical conductor consisting of a flexible tube made of a transparent thermoplastic material filled with a liquid acting as the light transfer medium. The tube particularly may be bent in the form of a circle. By coloring the liquid, the emitted illuminating light may be adjusted independently of the source of light. Alternatively, a color filter may be provided at one end of the optical conductor. No continuous color control is provided.